The present invention in general relates to the determination of the charge condition of a battery on the basis of the terminal voltage of said battery, particularly a rechargeable battery.
The present invention more particularly, though not exclusively, relates to Li-ion batteries, and the invention will be explained specifically for this example of use. However, it is emphasized that the present invention likewise relates to other battery types.
Nowadays many electrical apparatuses are powered by means of a battery. During operation of such an apparatus the battery is gradually discharged until it is finally empty. For many apparatuses it is desirable that the user is given an indication of the instantaneous charge condition of the battery in order not be unexpectedly confronted with an empty battery, as a result of which the apparatus ceases to function. Some important examples of apparatuses for which this is very desirable are video cameras, laptop computers and mobile telephones. It is usually adequate to have a coarse indication of the charge condition of the battery, for example in three or four gradations. However, in some cases it is desirable to have an accurate read-out of the charge condition, for example in per cent of the maximum charge or in residual playing time expressed in minutes or even seconds. Although the present invention will be elucidated specifically for the case of a mobile telephone, the same issue holds for other uses, so that the use of the invention has a broader scope.
It is to be noted that herein the term xe2x80x9ccharge conditionxe2x80x9d in general is to be understood to means the amount of charge or energy present in the relevant battery. This charge or energy can be expressed in absolute terms or in relative terms as a fraction of the maximum. In this case 0% corresponds to an empty battery and 100% corresponds to a fully charged battery. However, it will be evident to one skilled in the art that it is alternatively possible to define the charge condition of a battery as the extent to which this battery has been drained, in which case 0% corresponds to a fully charged battery and 100% corresponds to an empty battery.
Various methods of determining the charge condition of a battery are known, which are based on the fact that the terminal voltage of most battery types depends on the charge condition. The terminal voltage decreases as the battery is drained further. Thus, in the known measurement methods the terminal voltage of the battery is always measured and an indication of the charge condition is derived from the measured terminal voltage.
However, in practice this measurement principle gives rise to some problems.
First of all, the terminal voltage depends not exclusively on the charge condition but also on other factors such as the temperature of the battery and the age of the battery, or the number of charging/discharging cycles the battery has undergone. When the battery is in use the terminal voltage also depends on the magnitude of the current supplied by the battery, which means that the impedance of the load and the internal impedance of the battery play a part. In order to eliminate the last-mentioned factors it is therefore better to measure the terminal voltage of the battery in the no-load condition but this is not always possible. Moreover, it takes some time after disconnection the load before the terminal voltage of the battery has reached a stable value (relaxation).
A further problem in this context is that a rechargeable battery for an apparatus such a s mobile telephone is accommodated in a battery pack whose contact faces correspond to contact faces of the application apparatus. As a rule, components such as a safety switch are arranged between the battery terminals and the contact faces of the battery pack, as a result of which a voltage loss occurs in this path. Furthermore, a contact resistance occurs between the battery pack and the apparatus, which gives rise to a voltage loss whose magnitude is unknown, is not constant in time, depends on unknown and variable factors such soiling of the contact faces, etc. Al this means that in practice it is not readily possible to measure the actual terminal voltage of the battery in the application apparatus.
Therefore, it is an objects of the present invention to provide a method of determining the charge condition of a battery, which mitigates or eliminates the aforementioned problems. More in particular, it is an object of the present invention to provide a method of determining the charge condition of a battery, which can be used in a loaded condition of a battery and which is largely independent of extraneous conditions.
The invention is based on the recognition of the fact that there is a relationship between the charge condition of a battery and the electromotive force of this battery, hereinafter also referred to as VEMF, which relationship is independent of extraneous conditions to a satisfactory extent .
According to the present invention ,on the basis of this recognition, a method of determining the charge condition of a battery is characterized in that the VEMF of the battery is determined and the charge condition is determined from the measured VEMF on the basis of said relationship.
A further aspect of the present invention is based on the recognition of the fact that the characteristic of VEMF versus the charge of a battery, at least for an Li-ion battery, has a number of stages with fairly sharp transitions between these stages, and that the positions of these stages is hardly or not dependent on variations in ambient factors. Thus, these transitions may be regarded as calibration points. On the basis of the recognition of this fact, according to the invention, a method of determining the charge condition of a battery is preferably characterized in that dVEMF/dQ is measured at regular intervals during charging and discharging, respectively, and the charge condition is determined also on the basis of the result of this measurement.